This one's going to be short. Want to let customers purchase stuff without signing up for a user account? Allow guest checkout by putting OSCAR_ALLOW_ANON_CHECKOUT = True into settings.py! Done.
After yesterday's frustrating process, I took a long hike up Šehitluci to chill out. When I came back down this cool conservationist dude about to cycle across Europe taught me how to identify edible herbs in a garden gone wild and I ate them. Then, I googled "javascript how to click element after event". A few googles later, I'd cobbled together the right code. I am so relieved this turned out easy.
This is the story of how I thought I'd spend an hour or so, maybe two, fixing it so that when people press the "Place Order" button on the /preview page, they get sent to the /thank-you page. I started at around 10-11am. I sort of worked it out at around 11.25pm.
Update 18 Apr 2018: Checkout Express doesn't automatically log customers out after payment. As far as I know, and according to this issue, there is presently no solution. What a nightmare scenario. Take this into consideration if you still want to use it.
I thought I'd need to override Oscar's checkout views to integrate Paypal. As it turns out, I don't have to. But, since I've already done it and figured out that it has a particular naughty quirk, I might as well publish this.